1. Field of the Invention
The present invention relates to a BAW resonator (BAW=bulk acoustic wave). In particular, the present invention relates to BAW resonators having a plurality of layers comprising different material orientations. In addition, the present invention relates to BAW filters comprising such BAW resonators.
2. Description of Prior Art
BAW filters comprising one or several BAW resonators, e.g. in a ladder-type circuit, have been known in the art. The BAW resonators used for these BAW filters are so-called thin-film BAW resonators, i.e. resonators comprising a piezoelectric thin film. The disadvantage of these prior art BAW filters is that no filter topology is known which converts signals from unbalanced/balanced signals to balanced/unbalanced signals without entailing restrictions with regard to the common-mode load impedance toward mass, or which can do without the additional coils or transformers/converters.
A further disadvantage of these prior art BAW filters is that they include, at frequencies of more than 5 GHz, piezolayers whose thicknesses for a fundamental-mode wave (fundamental-mode BAW) are extremely thin (<300 nm). A further disadvantage is that at such frequencies of more than 5 GHz, those resonators which have a predetermined impedance level are smaller than is desired for performance reasons, since this yields, for example, a poor ratio of area and circumference of the arrangement, which leads to strong parasitic effects.
Yet another disadvantage of the prior art BAW filter is the fact that the thickness of a piezolayer for a fundamental-mode wave (fundamental-mode BAW) will be quite thick (>5 μm) at frequencies below 500 MHz. This leads to the added disadvantage that considering a dielectric constant of 10 (of the substrate), a respective individual resonator having an impedance level of 50 ohm will require an area of >0.5 mm2.
Even though in the prior art solutions have been known by means of which the problem of converting balanced/unbalanced signals into unbalanced/balanced signals is made possible, these solutions, too, pose the above-mentioned problems in connection with the common-mode load impedance toward mass, and/or in connection with the use of additional devices.
The prior art has known solutions for filter arrangements for frequencies above 5 GHz, but it is cavity resonators or ceramic resonators that are typically used for this purpose, which are both rather bulky, lossy in terms of electricity and very expensive.
For frequency ranges of up to 200 MHz, quartz-crystal resonators, whose highest operating frequency nowadays is 200 MHz, have been known in the prior art. Filter operations in the range from 100 MHz to 2 GHz are performed mainly using surface acoustic wave filters (SAW Filters), which have the drawback that they are rather bulky and are, in addition, very expensive in the range of less than 500 MHz.
In addition, stacked crystal-resonator structures have been known in the art. In this context, reference shall be made to the article “Stacked Crystal Filter Implemented with Thin Films” by K. M. Lakin et al., 43rd Annual Symposium on Frequency Control (1989), pages 536-543.